Electrically-variable transmissions typically have an input member, an output member, and two electric motor/generators connected to different members of planetary gear sets. Including clutches allows one or more electrically-variable modes of operation, fixed speed ratio modes, and an electric-only (battery-powered) mode. Electrically-variable transmissions may improve vehicle fuel economy in a variety of ways, primarily by using one or both of the motor/generators for vehicle braking and using the regenerated energy to power the vehicle electrically, with the engine off. The engine may be turned off at idle, during periods of deceleration and braking, and during periods of low speed or light load operation to eliminate efficiency losses due to engine drag. Braking energy captured via regenerative braking (or electrical energy generated by during periods when the engine is operating) is utilized during these engine-off periods. Transient demand for engine torque or power is supplemented by the motor/generators during operation in engine-on modes, allowing for a smaller engine without reducing vehicle performance. Additionally, the electrically-variable modes may allow the engine to be operated at or near the optimal efficiency point for a given power demand.
It is challenging to package the two motor/generators, planetary gear sets, and multiple torque-transmitting mechanisms necessary to achieve the desired modes of operation, while meeting other applicable dimensional limitations and achieving relatively simple assembly requirements. Furthermore, although electrically variable transmissions are being mass-produced today, they are installed in only a very small fraction the total number of vehicles produced today, and so engineering and tooling costs are much more important as compared with those costs for other types of transmissions.